As the recent years had shown, major social sites have a serious influence on the world. However, it is not just about size.

It is also about who are your users: The economist is not widely read, but it is read by the movers and shakers everywhere.

I like to picture HN as a kind of The Economist of "hackers" (influence-wise)

If we believe "hackers" are essential for the "good" of the "world", AND we care about the "World", then we should care about HN.